1. Field of the Invention
The present invention relates to a method of detecting the thickness of a thin layer and an apparatus therefor, and more particularly, to a method of detecting the thickness of a thin layer formed on the surface of a semiconductor wafer, and an apparatus therefor.
2. Description of the Related Art
As the level of integration of semiconductor devices increases, the thickness of a thin layer formed on a wafer is an important parameter that must be controlled in the manufacture of semiconductor devices. Failure to adjust the thickness of a thin layer to within a desired range causes degradation of the devices, and this degradation reduces the margins in a photolithography process, thereby causing a deterioration in the yield. Thus, precise and accurate detection of the thickness of the top layer over a specified portion of the surface of a wafer is considerably important.
The conventional optical detection method suffers some drawbacks. The actual detection of the thicknesses of patterned cells to be measured is difficult due to the optical interference of those patterns beneath the layer to be measured. This difficulty is exacerbated by certain measurement limitations/restrictions depending on the sizes of beam spots projected to the patterns. Hence, in the conventional detection method, the only instances where layer thickness can be accurately ascertained is for flat structures having no patterns beneath the layer to be measured.
In order to achieve such a detection, as shown in FIGS. 1 and 2, the thickness of a thin layer formed on a wafer 1, particularly, the thickness of the top layer 2 formed over cells B in which predetermined lower patterns 3 are formed, is detected from the thickness of an unpatterned adjacent oxide site A between the cells B, during a semiconductor manufacturing process. The size of the oxide site A is about 20xc3x9720 to 100xc3x97200 m2 in consideration of the size of the beam spot. As is readily seen, in the conventional thickness detection method, the thickness of a thin layer 2 formed over cells B must be measured indirectly by estimating the thickness of a thin layer in the adjacent oxide site A, which means the thickness cannot be accurately detected. Thus, a vertical secondary electron microscope (VSEM) is employed to overcome a detection limit due to the above-described non-destructive thickness detection method. However, the use of the VSEM causes the breakdown of a wafer, thus preventing real time detection of the thickness. Moreover, with the above described measurement method, a large amount of analysis time is required to measure the thickness of a target.
An object of the present invention is to provide a method of accurately detecting the thickness of a thin layer by which the thickness of the top layer formed on the surface of a wafer can be non-destructively detected. An apparatus for performing such a method is also provided.
Another object of the present invention is to provide a method of detecting the thickness of a thin layer by which the thickness of the top layer formed over cell areas having patterns can be directly detected.
Still another object of the present invention is to provide a method of detecting the thickness of a thin layer by which the thickness of the top layer formed over cell areas can be non-destructively detected in real time.
Accordingly, to achieve the first object, the present invention provides a method of detecting the thickness of a thin layer, including the steps of: (a) irradiating light toward a cell formed on a wafer and obtaining a luminance value from reflected light reflecting from the cell; (b) detecting a thickness of a thin layer in an oxide site positioned adjacent to the cell; (c) repeating the steps (a) and (b) a plurality of times to obtain a plurality of luminance values from respective cells formed on the wafer and a plurality of thickness values of thin layers in oxide sites interposed between adjacent of the respective cells; (d) employing a thickness calculation formula for calculating the thickness of a thin layer using the plurality of luminance values and the plurality of thickness values obtained in the step (c); and (e) obtaining the calculated thickness of a top layer in a cell using the thickness calculation formula employed in the step (d).
In this present invention, it is preferable that light incident upon the cell is white light. It is also preferable that the luminance of light reflected by the cell is detected by dividing the reflected light into red and/or green light and obtaining the luminance of each divided light.
Alternatively, light incident upon the cell may be monochromatic light, for example, red light or green light. In this case, the luminance of reflected green or red light can be directly obtained without dividing the reflected light into light of specific colors.
To accomplish the above objects, the present invention also provides an apparatus for detecting the thickness of a thin layer, including: a light source for irradiating light onto a wafer provided with cells having predetermined lower patterns, and an oxide site interposed between adjacent cells; a camera for receiving light reflected by the cells of the wafer; a luminance measurer for measuring the luminance from a picture signal output by the camera; and a thin layer thickness calculator for calculating the thickness of a top layer in the cells using the luminance obtained by the luminance measurer.
Here, it is preferable that the thin layer thickness calculator calculates the thickness of the top layer in the cells of the wafer by applying the luminance of light reflected by a plurality of cells on a sample wafer which have the same structure as the cell of the detection target wafer, and the thickness values of oxide sites that are adjacent to each cell.
Also, the light source irradiates white light or monochromatic light, preferably, green light or red light. When the light source irradiates white light, the color luminance measurer divides the white light into green and/or red light and calculates the average luminance of each color light. On the other hand, when the light source irradiates monochromatic light, the color luminance measurer directly obtains the average luminance of the monochromatic light. The thickness calculator calculates the thickness of the top layer in a cell by substituting the received mean luminance values for each color in the thickness calculation formula.